The present invention relates to a method for producing a security paper or board product or a security package carrying micro or nano structures such as diffractive optical elements for providing information for authentication. The present invention also relates to a security package containing authentication information in a form of diffractive structures.
Diffractive optical elements are optical components, which contain structures with dimensions of the order of wavelength of light. With diffractive optical elements it is possible to control propagation of light by macroscopically smooth surfaces containing micro or nano structures. These structures are later referred to as diffractive structures. A simple example of a diffractive optical component is a one-dimensional diffraction grating consisting of periodic grooves of the order of wavelength. When a white light beam is passed through or reflected from a diffraction grating it is dispersed in a spectrum. “Diffractive Optics for Industrial and Commercial Applications” (edited by Turunen and Wyrowski, Akademie Verlag 1997, ISBN 3-05-501733-1) discloses diffractive optics and components and their use and design. This source is later referred to as “Diffractive Optics”.
It is known from prior art to use diffractive structures as well as other optical security elements as watermarks in valuable documents and products for authentication purpose. Diffractive optical security elements are typically embossed on thin foils and applied on articles to be marked as separate adhesives. Optical security objects can include various elements observable by various methods. According to “Diffractive Optics” these observation methods can be divided into first-line, second-line, and third-line inspection levels. First-line inspection is based on the human senses only, for instance vision, hearing and the tactile senses. Members of the general public, who must be able to distinguish counterfeits and forgeries, mainly use first-line inspection that can be performed by the naked eye. In second-line inspection, simple tools are used to reveal hidden security objects. Examples of these tools are magnifiers, barcode scanners, laser pointers, ultraviolet sources and automatic teller machines. Third-line inspection involves forensic investigation of the security element performed by experts using advanced techniques and equipment. This kind of equipment is very expensive and is available only in a few research institutes in the world.
Manufacturing of diffractive structures requires advanced and complex systems, which only are available in advanced laboratories. Therefore diffractive structures are very difficult to forge.
According to “Diffractive Optics”, advantages for using diffractive structures as security elements are firstly, that they can not be reproduced with colour copiers or modern desktop publishing equipment. Secondly, production of diffractive structures is quite involved and it requires special equipment and knowledge. Thirdly, the optically variable effects are generally quite noticeable and therefore they adequately facilitate first line inspection.
Diffractive structures are usually manufactured by microlithographic methods. U.S. Pat. No. 4,662,653 discloses an optically diffracting security element comprising a continuous reflecting surface, a dielectric layer formed contiguous with the reflecting surface, and a plurality of non-continuous reflecting surface portions embedded in the dielectric layer in a predetermined arrangement for storing authentication information and a process for forming such element.
U.S. Pat. No. 5,862,750 discloses a method for impression microengravings, which reproduce holograms, kinetic holograms or diffraction patterns, directly on paper through an embossing process. In this method paper is subjected to a pre-treatment step prior to embossing said microengravings to paper. The required pre-treatment is a humidification step, which gives to a paper a degree of humidity between 60% and 80% of relative humidity. The humidified paper is then passed through an embossing group at a certain temperature and pressure. According to said publication it is not possible to impress microengravings directly on untreated paper.
U.S. Pat. No. 5,871,615 discloses security paper carrying a surface profile pattern imparted to the paper during its manufacture which requires de-watering of the paper when imparting the profile pattern and drying thereafter. The tactile surface profile pattern is visible when viewed under low angle light, which facilitates verification or authentication of security documents made using the patterned paper.
U.S. Pat. No. 5,981,040 discloses a holographic image produced of resinous ink comprising metallic particles. This special ink is used for printing to a sensitive document to form a reflective coating, which is embossed by a shim. The embossed area of the reflective coating reflects light in a slightly different direction than the remainder of the reflective coating, thereby creating a holographic image.
U.S. Pat. No. 5,974,150 discloses an authentication system comprising a medium having a plurality of elements, which are distinctive, detectable and disposed in an irregular pattern or having an intrinsic irregularity. The system provides authentication of an object by providing at least two levels of security, which are a physical level, provided by an observable feature an authentication certificate, and an information level, provided by encoding a unique characteristic of the authentication certificate (such as the observable feature) and/or object to be authenticated in a marking on the certificate.
U.S. Pat. No. 5,961,152 discloses security paper which has a filament bonded and embedded into paper which has been previously manufactured. The filament is bonded to the paper by an adhesive, or by heat and pressure. The filament may include a combination of security features, such as reflective filaments, fluorescent filaments, and high tensile strength filaments.
The problems with the prior art security objects are that they are expensive to manufacture, difficult or expensive to integrate on paper and easy to counterfeit if they are applicable as separate adhesive labels. The same applies to laminated package materials containing diffractive foil layers.